Information About Remote Access IPsec VPNs

Remote access VPNs allow users to connect to a central site through a secure connection over a TCP/IP network such as the Internet. The Internet Security Association and Key Management Protocol, also called IKE, is the negotiation protocol that lets the IPsec client on the remote PC and the adaptive security appliance agree on how to build an IPsec Security Association. Each ISAKMP negotiation is divided into two sections called Phase1 and Phase2.

Phase 1 creates the first tunnel to protect later ISAKMP negotiation messages. Phase 2 creates the tunnel that protects data travelling across the secure connection.

To set the terms of the ISAKMP negotiations, you create an ISAKMP policy. It includes the following:

•An authentication method, to ensure the identity of the peers.

•An encryption method, to protect the data and ensure privacy.

•A Hashed Message Authentication Codes (HMAC) method to ensure the identity of the sender and to ensure that the message has not been modified in transit.

•A Diffie-Hellman group to set the size of the encryption key.

•A time limit for how long the adaptive security appliance uses an encryption key before replacing it.

A transform set combines an encryption method and an authentication method. During the IPsec security association negotiation with ISAKMP, the peers agree to use a particular transform set to protect a particular data flow. The transform set must be the same for both peers.

A transform set protects the data flows for the access list specified in the associated crypto map entry. You can create transform sets in the adaptive security appliance configuration, and then specify a maximum of 11 of them in a crypto map or dynamic crypto map entry. For more overview information, including a table that lists valid encryption and authentication methods, see Creating a Transform Set in Chapter 69 "Configuring LAN-to-LAN IPsec VPNs" of this guide.

Licensing Requirements for Remote Access IPsec VPNs

The following table shows the licensing requirements for this feature:

1Although the maximum IPSec and SSL VPN sessions add up to more than the maximum VPN sessions, the combined sessions should not exceed the VPN session limit. If you exceed the maximum VPN sessions, you can overload the adaptive security appliance, so be sure to size your network appropriately.

Guidelines and Limitations

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single context mode only. Does not support multiple context mode.

Configuring Interfaces

An adaptive security appliance has at least two interfaces, referred to here as outside and inside. Typically, the outside interface is connected to the public Internet, while the inside interface is connected to a private network and is protected from public access.

To begin, configure and enable two interfaces on the adaptive security appliance. Then assign a name, IP address and subnet mask. Optionally, configure its security level, speed and duplex operation on the security appliance.

To configure interfaces, perform the following steps, using the command syntax in the examples:

Detailed Steps

Command

Purpose

Step 1

interface {interface}

Example:

hostname(config)# interface ethernet0

hostname(config-if)#

Enters interface configuration mode from global configuration mode.

Step 1

ip addressip_address [mask] [standbyip_address]

Example:

hostname(config)# interface ethernet0

hostname(config-if)#

hostname(config-if)# ip address
10.10.4.200 255.255.0.0

Sets the IP address and subnet mask for the interface.

Step 2

nameifname

Example:

hostname(config-if)# nameif outside

hostname(config-if)#

Specifies a name for the interface (maximum of 48 characters). You cannot change this name after you set it.

Step 3

shutdown

Example:

hostname(config-if)# no shutdown

hostname(config-if)#

Enables the interface. By default, interfaces are disabled.

Configuring ISAKMP Policy and Enabling ISAKMP on the Outside Interface

This section describes the procedure to configure an ISAKMP policy on the outside interface and how to enable the policy.

Detailed Steps

Perform the following steps and use the command syntax in the following examples as a guide.

Command

Purpose

Step 1

isakmp policy priority authentication
{crack | pre-share | rsa-sig}

Example:

hostname(config)# isakmp policy 1
authentication pre-share

hostname(config)#

Specifies the authentication method and the set of parameters to use during IKE negotiation.

Priority uniquely identifies the Internet Key Exchange (IKE) policy and assigns a priority to the policy. Use an integer from 1 to 65,534, with 1 being the highest priority and 65,534 the lowest.

Specifies the hash algorithm for an IKE policy (also called the HMAC variant).

Step 4

isakmp policy priority group
{1 | 2 | 5}

Example:

hostname(config)# isakmp policy 1 group 2

hostname(config)#

Specifies the Diffie-Hellman group for the IKE policy—the crypto protocol that allows the IPsec client and the adaptive security appliance to establish a shared secret key.

Step 5

isakmp policy priority lifetime {seconds}

Example:

hostname(config)# isakmp policy 1
lifetime 43200

hostname(config)#

Specifies the encryption key lifetime—the number of seconds each security association should exist before expiring.

The range for a finite lifetime is 120 to 2147483647 seconds. Use 0 seconds for an infinite lifetime.

Step 6

isakmp enable interface-name

Example:

hostname(config)# isakmp enable outside

hostname(config)#

Enables ISAKMP on the interface named outside.

Step 7

write memory

Example:

hostname(config-if)#write memory

Building configuration...

Cryptochecksum: 0f80bf71 1623a231 63f27ccf
8700ca6d

11679 bytes copied in 3.390 secs (3893
bytes/sec)

[OK]

hostname(config-if)#

Saves the changes to the configuration.

Configuring an Address Pool

The adaptive security appliance requires a method for assigning IP addresses to users. This section uses address pools as an example. Use the command syntax in the following examples as a guide.

Command

Purpose

ip local pool poolname
first-address—last-address[mask mask]

Example:

hostname(config)# ip local pool testpool
192.168.0.10-192.168.0.15

hostname(config)#

Creates an address pool with a range of IP addresses, from which the adaptive security appliance assigns addresses to the clients.

The address mask is optional. However, You must supply the mask value when the IP addresses assigned to VPN clients belong to a non-standard network and the data could be routed incorrectly if you use the default mask. A typical example is when the IP local pool contains 10.10.10.0/255.255.255.0 addresses, since this is a Class A network by default. This could cause routing issues when the VPN client needs to access different subnets within the 10 network over different interfaces.

Adding a User

This section shows how to configure usernames and passwords. Use the command syntax in the following examples as a guide.

Configures a transform set that specifies the IPsec encryption and hash algorithms to be used to ensure data integrity.

Use one of the following values for encryption:

•esp-aes to use AES with a 128-bit key.

•esp-aes-192 to use AES with a 192-bit key.

•esp-aes-256 to use AES with a 256-bit key.

•esp-des to use 56-bit DES-CBC.

•esp-3des to use triple DES algorithm.

•esp-null to not use encryption.

Use one of the following values for authentication:

•esp-md5-hmac to use the MD5/HMAC-128 as the hash algorithm.

•esp-sha-hmac to use the SHA/HMAC-160 as the hash algorithm.

•esp-none to not use HMAC authentication.

Defining a Tunnel Group

This section describes how to configure a tunnel group, which is a set of records that contain tunnel connection policies. You configure a tunnel group to identify AAA servers, specify connection parameters, and define a default group policy. The adaptive security appliance stores tunnel groups internally.

There are two default tunnel groups in the adaptive security appliance system: DefaultRAGroup, which is the default IPsec remote-access tunnel group, and DefaultL2Lgroup, which is the default IPsec LAN-to-LAN tunnel group. You can change them but not delete them. The adaptive security appliance uses these groups to configure default tunnel parameters for remote access and LAN-to-LAN tunnel groups when there is no specific tunnel group identified during tunnel negotiation.

Enters tunnel group general attributes mode where you can enter an authentication method.

Step 3

address-pool [(interface name)]
address_pool1 [...address_pool6]

Example:

hostname(config-general)# address-pool
testpool

Specifies an address pool to use for the tunnel group.

Step 4

tunnel-groupnameipsec-attributes

Example:

hostname(config)# tunnel-group testgroup
ipsec-attributes

hostname(config-tunnel-ipsec)#

Enters tunnel group ipsec attributes mode where you can enter ipsec-specific attributes.

Step 5

pre-shared-keykey

Example:

hostname(config-tunnel-ipsec)#
pre-shared-key 44kkaol59636jnfx

(Optional) Configures a pre-shared key. The key can be an alphanumeric string from 1-128 characters.

The keys for the adaptive security appliance and the client must be identical. If a Cisco VPN Client with a different preshared key size tries to connect, the client logs an error message indicating it failed to authenticate the peer.

Creating a Dynamic Crypto Map

This section describes how to configure dynamic crypto maps, which define a policy template where all the parameters do not have to be configured. These dynamic crypto maps let the adaptive security appliance receive connections from peers that have unknown IP addresses. Remote access clients fall in this category.

Dynamic crypto map entries identify the transform set for the connection. You also enable reverse routing, which lets the adaptive security appliance learn routing information for connected clients, and advertise it via RIP or OSPF.

Detailed Steps

Creates a dynamic crypto map and specifies a transform set for the map.

Step 2

crypto dynamic-map dynamic-map-namedynamic-seq-num set reverse-route

Example:

hostname(config)# crypto dynamic-map dyn1
1 set reverse route

hostname(config)#

(Optional) Enables Reverse Route Injection for any connection based on this crypto map entry.

Creating a Crypto Map Entry to Use the Dynamic Crypto Map

This section describes how to create a crypto map entry that lets the adaptive security appliance use the dynamic crypto map to set the parameters of IPsec security associations.

In the following examples for this command, the name of the crypto map is mymap, the sequence number is 1, and the name of the dynamic crypto map is dyn1, which you created in the previous section, "Creating a Dynamic Crypto Map."

Use the command syntax in the following examples as a guide.

Detailed Steps

Command

Purpose

Step 1

crypto map map-name seq-num ipsec-isakmp
dynamic dynamic-map-name

Example:

hostname(config)# crypto map mymap 1
ipsec-isakmp dynamic dyn1

hostname(config)#

Creates a crypto map entry that uses a dynamic crypto map.

Step 2

crypto mapmap-nameinterfaceinterface-name

Example:

hostname(config)# crypto map mymap
interface outside

hostname(config)#

Applies the crypto map to the outside interface.

Saving the Security Appliance Configuration

After performing the preceding configuration tasks, be sure to save your configuration changes as shown in this example:

Command

Purpose

write memory

Example:

hostname(config-if)#write memory

Building configuration...

Cryptochecksum: 0f80bf71 1623a231 63f27ccf 8700ca6d

11679 bytes copied in 3.390 secs (3893 bytes/sec)

[OK]

hostname(config-if)#

Saves the changes to the configuration.

Configuration Examples for Remote Access IPsec VPNs

The following example shows how to configure Remote Access IPsec VPNs: